1. Technical Field
Various embodiments relate to a wireless communication apparatus, and more particularly, to a multi-input multi-output (MIMO) repeater system and a method for canceling feedback interference using the same.
2. Related Art
In general, a repeater is used in a mobile communication system, in order to expand the coverage of the mobile communication system in a shadow area where the intensity of radio waves from a base station decreases or an area remote from the base station.
The repeater includes an optical repeater using an optical cable and a wireless repeater which wirelessly receives a signal from a base station and wirelessly transmits the received signal.
The optical repeater guarantees stable performance. However, since the optical cable must be installed or rented, the cost inevitably increases. Since the wireless repeater does not need an optical cable, an installation place may be freely selected, maintenance and repair may be easily performed, and the frequency may be reused.
FIG. 1 is a conceptual view of a conventional wireless repeater.
A repeater 3 receives a signal which is wirelessly transmitted from a base station 1. The repeater 3 amplifies the received signal and then transmits the amplified signal as a wireless signal to a terminal 5.
Since the received signal and the transmitted signal of the wireless repeater use the same frequency, isolation between transmit (Tx) and receive (Rx) antennas must be sufficiently secured. Otherwise, a signal transmitted from the Tx antenna may be fed back to the Rx antenna. Then, since the original signal transmitted from the base station 1 is coupled to the feedback signal, oscillation may occur.
In order to solve such a problem, an interference cancellation system (ICS) capable of canceling feedback interference may be employed. FIG. 2 illustrates an example of the ICS.
Referring to FIG. 2, the ICS repeater 10 having a feedback interference cancellation function includes an Rx antenna 101, a first signal processing unit 103, an interference cancellation unit 105, a second signal processing unit 107, and a Tx antenna 109.
The Rx antenna 101 is configured to receive a radio frequency (RF) signal from a base station. The first signal processing unit 103 is configured to convert the RF signal received through the Rx antenna 101 into a baseband digital signal.
The interference cancellation unit 105 is configured to estimate and cancel an interference signal from the output signal of the first signal processing unit 103 and then output the signal.
The second signal processing unit 107 is configured to adjust the frequency band of the signal, from which the feedback interference is canceled by the interference cancellation unit 105, to a high-frequency band, convert the adjusted signal into an analog signal, and transit the analog signal through the Tx antenna 109.
The ICS repeater 10 is a system capable of estimating and canceling a feedback interference signal received by a repeater. When the Tx and Rx antennas can be isolated at a sufficient distance from each other, the ISC repeater does not need to be used. However, since an indoor repeater for domestic use cannot sufficiently secure isolation between the Tx and Rx antennas, the ISC repeater is used to prevent oscillation of the repeater. Furthermore, since the ICS repeater has a low installation cost, the ICS repeater is preferred in mountain and island areas.
The ICS repeater is designed under the supposition that a single antenna is used. However, in third-generation mobile communication systems or fourth-generation mobile communication systems such as LTE (Long Term Evolution) and Mobile WiMax, a base station and a terminal use multiple antennas. Thus, a repeater uses multiple antennas to repeat a signal.
FIG. 3 is a schematic view of a multi-input multi-output (MIMO) repeater system.
The MIMO repeater system 20 includes a plurality of Rx antennas 201, a repeater 203, and a plurality of Tx antennas 205. The repeater 203 is configured to repeat signals of the Rx antennas 201, and the plurality of Tx antennas 205 are configured to transmit the repeated signals in a wireless manner. The respective Tx antennas 205 transmit different signals to a terminal.
In such a MIMO repeater system, since different signals transmitted from the plurality of Tx antennas 205 are fed back to the respective Rx antennas 201, signals fed back to one Rx antenna may include a signal transmitted from a Tx antenna connected to the Rx antenna and signals transmitted from other Tx antennas.
Thus, there is an urgent demand for a feedback interference cancellation technique for the MIMO repeater system.